One method of combining multiple measurements from a sensor or a suite of sensors is to use an evidence grid. An evidence grid, also called an occupancy matrix, is merely a two- or three-dimensional matrix of cells, which are to be marked as either occupied or not occupied. The resulting matrix of occupied and unoccupied cells serves as a representation of the real-world scene that the sensors are sensing.
The usual method of filling out an evidence grid is to take each measurement and determine from it whether cells in the grid are occupied or not. As subsequent measurements are made, the cells in the grid are updated, but not necessarily with regard to the results of previous measurements. As a result, there is no real meaningful combining of the data from multiple measurements, nor is there any real method of combining measurements from different types of sensors.
FIG. 1 illustrates in two dimensions a three-dimensional volume 10 detected as a result of a single signal transmitted by a radar or other sensing device (not shown). As detected by the sensing device, the volume 10 includes an occupied region 20 indicating the detected presence therein of a reflecting object. The volume 10 further includes an unoccupied region 30 within which no reflecting object is detected. Analysis of this single radar return cannot effectively use the information associated with the unoccupied region 30.
FIG. 2 illustrates in two dimensions the volume 10 and a three-dimensional volume 40 detected as a result of a second signal transmitted by a radar or other sensing device (not shown). As detected by the sensing device, the volume 40 includes an occupied region 50, closer to the sensing device than the region 20, indicating the detected presence therein of a reflecting object. The volume 40 further includes an unoccupied region 60 within which no reflecting object is detected. The combination of the volumes 10 and 40 reveals a region 70 wherein no object seems to be present, despite the indication of the region 50. As such, by combining these two radar returns, one can better identify volumes of the target regions as being occupied or not.
FIG. 3 illustrates in two dimensions the volumes 10, 40 and a three-dimensional volume 80 detected as a result of a third signal transmitted by a radar or other sensing device (not shown) and superimposed on a corresponding evidence grid 85. As detected by the sensing device, the volume 80 includes an occupied region 90, farther from the sensing device than the regions 20, 50, indicating the detected presence therein of a reflecting object. The volume 80 further includes an unoccupied region 100 within which no reflecting object is detected. The combination of the volumes 10, 40 and 80 reveals that portions of the region 50 offer conflicting information as to whether the region 50 is occupied by an object. Such conflicting information may result from sensor or navigation errors, or moving targets, that lead to indeterminate points that are neither occupied nor unoccupied.